<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <div id="app"></div>
    <script>
        // 1、将当前的时间显示在div上
        function fn() {
            var d = new Date();
            var hours = d.getHours();
            var minutes = d.getMinutes();
            var seconds = d.getSeconds();
            app.innerHTML = hours + ':' + minutes + ':' + seconds;
        }

        fn();
        var timer = setInterval(fn,1000);
    </script>
</body>

</html>